1. Field of the Invention
The present invention relates to a substrate for an information recording medium which substrate contains an alkali metal oxide but causes remarkably less elution of alkali metal ion, an information recording medium having the above substrate and a process for producing the information recording medium. Particularly, it relates to a substrate for a magnetic recording medium which substrate is suitable for forming a perpendicular-magnetic-recording-mode magnetic layer with a high-temperature sputtering machine, and a magnetic recording medium and a process for producing the magnetic recording medium.
2. Prior Art
Conventionally, aluminum, glass, ceramics, or the like, is used as a substrate material for a magnetic recording medium. At present, aluminum and glass are practically used depending upon a size and use. Of these, the use of a glass substrate is expanding year after year since the glass substrate has fewer surface defects and is excellent in smoothness and surface hardness. As a glass for use as a substrate for a magnetic recording medium, there are known a chemically strengthened glass based on ion exchange and a crystallized glass. Concerning a chemically strengthened glass, there is known a glass substrate for a magnetic disk, which glass substrate is formed by chemically strengthening a glass containing, by % by weight, 50 to 85% of SiO2, 0.5 to 14% of Al2O3, 10 to 32% of R2O (R is alkali metal ion), 1 to 15% of ZnO and 1.1 to 14% of B2O3 by an ion exchange method using alkali ion and has a compression stress layer formed in a glass substrate surface (for example, see JP-A-1-239036). Concerning a crystallized glass, there is known a crystallized glass substrate for a magnetic disk, which substrate contains, by % by weight, 65 to 83% of SiO2, 8 to 13% of Li2O, 0 to 7% of K2O, 0.5 to 5% of MgO, 0 to 5% of ZnO, 0 to 5% of PbO, the total content of MgO+ZnO+PbO being 0.5 to 5%, 1 to 4% of P2O5, 0 to 7% of Al2O3 and 0 to 2% of As2O3+Sb2O3 and contains fine Li2O—2SiO2 crystal grains as a main crystal (for example, see Patent U.S. Pat. No. 5,391,622).
With the progress of higher-density recording in recent years, however, it is demanded that a longitudinal recording method should be converted to a perpendicular magnetic recording method for an information recording device such as a magnetic disk typified by a hard disk. In the longitudinal magnetic recording method, a domain easily rotates under heat of a room temperature degree, and it is therefore pointed out that when the recording density increases, writing is no longer possible, so that written data is easily lost. The above phenomenon is known as a thermal fluctuation problem and is coming to be an obstacle to a longitudinal magnetic recording method. For coping with the thermal fluctuation problem in the longitudinal magnetic recording method, the perpendicular magnetic recording method has been actively studied in recent years for its practical use.
With regard to the layer constitution of the above perpendicular magnetic recording method, there are known a single-layer film having a perpendicular magnetic recording layer formed on a non-magnetic substrate, a double-layer film having a soft magnetic layer and a magnetic recording layer stacked consecutively and a three-layer film having a hard magnetic layer, a soft magnetic layer and a magnetic recording layer stacked consecutively. Of these, double-layer films and three-layer films are vigorously developed in recent years for practical use, since they are more suitable for higher-density recording and maintenance of magnetic moment stability than the single-layer film. And, for improving the properties of the above multi-layered-film-applied magnetic perpendicular magnetic recording media, it is said that it is required to form the films with a high-temperature sputtering machine and to heat-treat the formed films at high temperatures.
However, an aluminum substrate that has been generally used so far has a low heat resistance of 280° C. Further, when a conventional chemically strengthened glass is used at a temperature in a chemical strengthening temperature range (350-420° C.) or a temperature higher than such a range, a surface stress caused by ion exchange for the chemical strengthening is relaxed, and the substrate strength sharply decreases, so that it is required to use a generally chemically strengthened glass at a temperature of 350° C. or lower. In the perpendicular magnetic recording method, it is required to form a film with a sputtering machine at a high temperature of 400° C. or higher or anneal the film of stacked layers at a temperature of 400° C. or higher, for improving a magnetic film in magnetic characteristic. It is therefore clear that none of aluminum substrates and chemically strengthened glass substrates commercially available at present can cope with the above requirement, and a glass substrate having high heat resistance is demanded.
For obtaining excellent heat resistance, a glass material that does not contain any alkali metal oxide so far essential for a chemically strengthened glass may be taken into account. However, in this case, there is caused a problem that the meltability of such a glass decreases since the glass viscosity during melting increases. When a magnetic disk is incorporated into a drive unit, it is required to impart a glass substrate with a larger thermal expansion coefficient for conforming the thermal expansion coefficient of the glass substrate to the counterpart of a metal fixing member to fix a disk. However, it is difficult to impart a glass containing no alkali metal oxide with the thermal expansion coefficient that a substrate for an information recording medium is required to have.
Further, when an alkali metal oxide is introduced for attaining excellent glass meltability and an excellent thermal expansion coefficient, there is a problem that an alkali is eluted from a glass substrate and has a detrimental effect on an information recording layer.
Conditions that a glass material for a substrate for an information recording medium is required to satisfy are summarized as follows.
{circle around (1)} High heat resistance
{circle around (2)} Excellent meltability
{circle around (3)} Low alkali-elution property